[clang] [clang] Update -Wformat warnings for fixed-point format specifiers (PR #82855)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Do you know a libc implementation that actually implements `%k` `%r` and who are the potential users? From a quick glance, gcc avr supports fixed-point types but avr-libc doesn't seem to support %k %r.

https://github.com/llvm/llvm-project/pull/82855